Automotive tires are safety relevant products. For liability reasons, there is an increasing demand to mark each tire individually in order to trace its production history in case of damages or claims. The markings must be human readable even after extended on-road usage of the tire. Markings consist of a series of alphanumeric symbols and have to be engraved on a well visible spot of the tire sidewall.
In the past, some tire manufacturers have used individual paper barcode labels on tires to control the production process of the tire. Such labels are stuck to the bead of each tire at the first production step and pass with the tire through the whole production chain up to final finishing. In this way, the individual information for each tire is available but not in a human readable fashion and not on a durable label.
German Utility Patent Nr. 203 10 931.7 describes a laser system that attempts to solve this marking task. In this system a conveyer belt transports the tire into a marking station until it stops underneath a horizontal swivel arm. Grip arms center the tire under the rotation axis of the swivel arm. A video camera and a laser are mounted onto the arm. The camera photographs the sidewall of the tire and is supposed to direct the laser to a marking spot after comparing the sidewall captured by the camera with photographs stored in memory. This concept does enable the use of very compact, inexpensive devices for tire marking. But video based picture-processing has revealed severe drawbacks. Video pictures rely on the contrast of colors (black/white). The raised symbols/letters on the tire sidewall are formed as part of the tire molding process. The symbols appear black on black, despite being in the form of raised figures. The human eye is only able to distinguish these black on black symbols because humans can unconsciously look for changes in the gloss on a tire surface. Simple video system processing, like that used in the above-described system, are not as sophisticated as human image processing capabilities and are unable to make such a distinction. Therefore slight changes on a tire's surface roughness or in workstation illumination can lead to malfunctions with prior systems.